1. Field of the Invention
This invention relates generally to the measurement of alerting or other signaling tones. More particularly, it relates to a versatile method and apparatus capable of accurately measuring a frequency of an alerting or other tone even in an environment including speech signals.
2. Background of Related Art
While the present invention relates to tone detection in general, it and its background are described with respect to a particular embodiment useful for detecting alerting tones relating to the reception of call related information, e.g., Caller ID information.
As is well known, Caller ID services permit a telephone customer to learn the identity of, or at least the originating telephone number of, a party seeking to place a telephone call to the customer. An older service referred to as Call-Waiting (CW) alerts a customer engaged in a telephone connection to another party, that a third party desires to place a telephone call to the customer. A newer service offered in telephone systems combines caller identification and call-waiting services, to not only alert a customer during a telephone connection that a third party desires to establish a connection, but also transmits to the CPE, caller identification information identifying the telephone number and/or name of the third party. This latter service is sometimes referred to as CIDCW (caller ID and call waiting) or Caller ID Type II.
Participation in a system with Caller ID and Call Waiting requires not only the presence of a central office capable of providing such service, but also CPE capable of receiving and transmitting the necessary control, acknowledgment and data signals. The general sequence of events during a telephone call in which the customer has Caller ID and Call Waiting service is as follows.
When a customer is currently engaged in a telephone conversation with a second party, a third party desiring to complete a telephone call with the customer may dial the customer's number. During the customer's conversation with the second party, the central office sends a subscriber alerting signal (SAS) alerting the customer that a third party desires to make a connection with the customer. The subscriber alerting signal is typically a single tone 400-hertz signal of about 500 milliseconds in duration, that is audible to the customer.
The central office follows the transmission of the subscriber alerting signal with the transmission of a CPE alerting signal (CAS). Similar to a dual tone multi-frequency signal, the CPE alerting signal has 2130 hertz and 2750 hertz components and optimally lasts for 80 milliseconds. Upon detection of the CPE alerting signal, the CPE sends a CAS acknowledgment signal (CAS ACK) to the central office, signaling to the central office that the CPE is ready to receive Caller ID information regarding the third party.
At the time the CPE transmits the CAS acknowledgment signal, it also disables the voice transmission transducer in the CPE. While the central office maintains the connection with the second party, it suppresses the CAS acknowledgment signal from the transmission sent to the second party. When the central office receives the CAS acknowledgment signal, it sends the caller identification information to the customer in the form of a frequency shift keyed (FSK) signal. The CPE can then display the caller identification information on a display unit as is known in the art. Full communication is established with the second party after receipt of the caller identification information. The customer, through the CPE can then decide how to handle the third party, such as placing the second party on hold and establishing the connection with the third party, for example.
Because of the proximity of the CPE alerting signal to voice frequencies transmitted and received in typical telephone systems, reliable detection of the CPE alerting signal can sometimes be a problem in conventional CPEs. For instance, conventional tone detectors used in this capacity sometimes experience "talkoff" episodes in which CPE alerting signals are falsely detected, and "talkdown" episodes in which actual CPE alerting signals are missed due to interference with speech. This erroneous operation is often the result of harmonic components of speech signals occurring in the frequency ranges of a tone signal such as the standard CPE alerting signal.
Many people have made telephone calls, enjoying the convenience of communications using a telephone or other customer premises equipment. Moreover, everyone has heard some type of alerting tone, call progress tone (CPT) or other signaling tone from a central office, be it a busy tone, a dial tone, or other single or dual frequency tone used to signal customer premises equipment or a central office over a telephone line.
Generally, alerting or other signaling tones are either continuous, periodic, intermittent, or single pulse only. The duration of any particular tone pulse may be very short, e.g., 40 milliseconds (mS) or less. Generally, a minimum length of a particular tone pulse is conventionally established to provide a conventional tone detection algorithm sufficient time to detect the presence of the tone.
FIG. 4 shows a conventional tone detector 902 adapted to detect the presence of a particular tone (i.e., a particular alerting or other signaling tone) in an input signal typically received over a telephone line, and output either a "tone detected" or a "tone not detected" signal. Many tone detectors contain a plurality of algorithms or parameters to accommodate the detection of a plurality of expected alerting tone or other tone signals. Some signaling tones require detection of more than one frequency component, e.g., Dual Tone Multiple Frequency (DTMF) signals.
Tone detectors (e.g., DTMF dialer and call progress tone detectors) are common in most customer premises equipment. Early customer premises equipment included analog filters for detecting a tone, but more recent customer premises equipment includes a processor with digital tone detection software. Digital tone detectors usually include an algorithm implementing a discrete Fourier transform (DFT).
A DFT processes digital samples of the input signal on a frame-by-frame basis. The DFT processes each sample frame to detect the presence of a particular frequency component in the input signal. If a particular frequency component above a predetermined energy threshold is detected in the processed frame, then a "tone detected" or similar signal is output.
Most conventional tone detectors detect the presence of tones when there is no speech on the telephone line, and thus most tone detectors need not be too robust to provide reliable results. Thus, short tones (i.e., tones having a minimum duration of, e.g., 40 mS) can be used for signaling. Moreover, in such systems, tone frequencies were sufficiently separated from each other to allow large tolerances in the detection and measurement of a tone in the input signal. More recently, it has become necessary to detect alerting and/or other tones in the presence of voice signals on a telephone line, and to measure their frequency with a high degree of accuracy.
FIG. 5 shows the approximate long-term average spectral energy density for continuous speech, showing that high relative spectral energy is present in near-end speech, which is in the same general range as many signaling tones used by central offices. Thus, the presence of speech together with an alerting or other tone may cause noise in the particular spectral regions being measured for the presence of a particular alerting or other signaling tone.
For instance, Call Subscriber Services such as Caller ID/Call Waiting (CIDCW) have become popular. Caller ID/Call Waiting is a Type II Caller ID service allowing the transmission of call related information such as a telephone number and/or household or business name to a called party regarding a third party caller to an established telephone call between the called party and someone else. Robust tone detectors are necessary to avoid the possibility of errors such as talkoff (false tone detects) and/or talkdowns (missed tone detects).
There is an increased need to accurately measure the frequency of a particular alerting or other signaling tone to increase robustness. Conventional alerting or other tones typically have a frequency offset or tolerance on the order of 2-3%. However, with the increase in Call Subscriber Services such as CIDCW and ADSL, there has become a need for more accurate frequency measurement and thus robust tone detectors, e.g., on the order of, e.g., 0.5-1% or less.
There is thus a need for a method and apparatus capable of more accurately measuring a frequency of an alerting or other signaling tone, even in the presence of speech or other distracting and/or distorting signals.